Space used does not match Disk Usage

We have a few customers who are complaining that their disk usage does not match their space used. By this I mean, they try to upload something, and the FTP server says that they are over their Quota, which is 300 megs. But if you look at their Disk Usage Viewer, they only show 40 megs of used space. I also ssh into the server, and do a du -sh /home/<username> and it says the same thing, 40 megs. So what do I need to do to make WHM not report false sizes?

Found the problem. If anyone else has this problem, look in /tmp/ and see if there are files in there that are owned by the user having the problem. Just delete those files, and the solves the problem.